The main long-term objective of the proposed research is a physical-chemical understanding of the processes mediating excitation and adaptation in vertebrate photoreceptors. We propose to test the hypothesis that those physiological processes are mediated by diffusible intracellular transmitter substances that are liberated when rhodopsin absorbs light and that act on specific ionic permeabilities of the plasma membrane. Specifically, we propose to examine the spatial distribution of the light-induced changes of intracellular ionized calcium, the putative messenger for light-adaptation in Limulus photoreceptors, and to determine the diffusion constant for calcium inside the photoreceptor cytosol. We also propose to examine the changes of intracellular concentrations of ions other than calcium using metallochromic indicators and a rapid-scanning microspectrophotometer. We propose also to use voltage-sensitive dyes to detect changes of membrane voltage of intracellular membranes in both vertebrate and invertebrate photoreceptors. In addition, we propose to investigate involvement of cyclic nucleotides in the process of excitation and adaptation by means of intracellular electrical recording and direct intracellular injection of relevant substances through the recording micropipettes.